If the rifle is of good quality and has the proper gas port size, I have never found any difference in reliability between the CAR and the Mid-length ARs.
There are reasons why this opinion has came about. That being companies putting in the wrong sized gas port in the barrel. For example the Mid-length requires a larger gas port hole in the barrel than the Car Length barrel referring to the 16" barrel for example. If they were to put too large a gas port in a Car application it would in all probability have too much gas and cause the rifle to run Hot! or out of time. This equals to feed and extraction issues. But should that occur most would say that the CAR was not reliable! This situation has occurred more times in the past in the industry than we would like to speak about! So the mystique of the Car not being as reliable has come about. Either, if built correctly and with the proper specifications is no less reliable than the other. You will hear the same argument when it comes to the gas impingement and gas piston systems for the same reasons.
I will admit the Gas Piston System keeps the inside of the receiver a little cleaner. So we are back to personal preference. I prefer the Mid-Length for two reasons. #1 Longer Sight Radius on the irons and #2 much more mounting surface if using a quad rail to mount accessories ie Lights, Optics, Vertical Grips and etc. I have both but like the Mid-Length the best. Take a look at the Rock River Arms Website and look at the CAR Model Govt.1000. You will see what I mean about the congestion of mounting a good light on the CAR length rifle compared to the Mid-Lengths pictured there. Good luck with your selection.